Suicide and suicide attempts cost an estimated $33 billion annually in the U.S. and immeasurable pain and suffering. Most suicidal behavior is associated with a depressive disorder, yet medication guidelines for suicidal patients are virtually nonexistent. The evidence base is severely limited because suicidal patients are excluded from most antidepressant clinical trials. Several recent studies show rapid improvement in suicidal ideation, in as little as one hour, in depressed patients after intravenous infusion of sub-anesthetic ketamine, a glutamate antagonist and commonly used anesthetic. The improvement in suicidal ideation appears to last several days, however, depression and suicidality are usually chronic, recurring conditions. We propose a rigorous clinical trial in suicidal depressed patients to test random assignment to IV infusion of ketamine or midazolam control followed by open continuation treatment with antidepressant medication plus supportive case management. The primary goal is to test ketamine's potential anti-suicidal effects versus a similarly sedative control medication not known to reduce suicidal ideation. Exploratory aims include analysis of potential biological (salivary cortisol) and neuro-cognitive correlates as well as systematic assessment of suicidal ideation and behavior during continuation treatment. Establishing a strong anti-suicidal signal for ketamine in this high-risk population would support an innovative intervention that could change clinical care for a patient population of public healt importance. Exploratory assessment of suicidal ideation and behavior during continuation treatment could help target research on strategies to sustain the benefit of ketamine. PUBLIC HEALTH RELEVANCE: Suicide is the 10th leading cause of death in the U.S. resulting in more than 30,000 deaths annually, but there is a surprising lack of evidence-based treatment for suicidal depression. Studies suggest rapid improvement in suicidal ideation in depressed patients within hours after a single, low-dose intravenous infusion of ketamine. We propose a rigorous trial to test the efficacy of ketamine in improving suicidal ideation in depressed patients with clinically significant suicidal ideation, a key population for the public health goal of suicide prevention, but which is usually excluded from clinical trials.